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Focus Formation: A Cell-based Assay to Determine the Oncogenic Potential of a Gene
Published on: December 31, 2014
Dynamic in vivo interactions among Myc network members
X Yin1, M F Landay, W Han
1Section of Hematology/Oncology, Department of Pediatrics, Children's Hospital of Pittsburgh, Pittsburgh, Pennsylvania, PA 15213, USA.
Abstract:
Members of the Myc oncoprotein network (c-Myc, Max, and Mad) play important roles in proliferation, differentiation, and apoptosis. We expressed chimeric green fluorescent protein (GFP) fusions of c-Myc, Max, and three Mad proteins in fibroblasts. Individually, c-Myc and Mad proteins localized in subnuclear speckles, whereas Max assumed a homogeneous nuclear pattern. These distributions were co-dominant and dynamic, however, as each protein assumed the pattern of its heterodimeric partner when the latter was co-expressed at a higher level. Deletion mapping of two Mad members, Mad1 and Mxi1, demonstrated that the domains responsible for nuclear localization and speckling are separable. A non-speckling Mxi1 mutant was also less effective as a transcriptional repressor than wild-type Mxi1. c-Myc nuclear speckles were distinct from SC-35 domains involved in mRNA processing. However, in the presence of co-expressed Max, c-Myc, but not Mad, co-localized to a subset of SC-35 loci. These results show that Myc network proteins comprise dynamic subnuclear structures and behave co-dominantly when co-expressed with their normal heterodimerization partners. In addition, c-Myc-Max heterodimers, but not Max-Mad heterodimers, localize to foci actively engaged in pre-mRNA transcription/processing. These findings suggest novel means by which Myc network members promote transcriptional activation or repression.
Insights
The Myc oncoprotein network proteins form dynamic structures within the nucleus. c-Myc and Max heterodimers, but not Max-Mad, localize to active transcription sites, suggesting novel regulatory roles.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncogenesis
Background:
- The Myc oncoprotein network, including c-Myc, Max, and Mad proteins, is crucial for cell proliferation, differentiation, and apoptosis.
- Understanding the subnuclear localization and dynamics of these proteins is key to elucidating their regulatory functions.
Purpose of the Study:
- To investigate the subnuclear localization and dynamics of Myc network proteins (c-Myc, Max, Mad1, Mxi1) in fibroblasts.
- To determine the functional significance of their subnuclear organization and heterodimerization on transcriptional regulation.
Main Methods:
- Expression of chimeric green fluorescent protein (GFP) fusions of c-Myc, Max, and Mad proteins in fibroblasts.
- Subnuclear localization studies using fluorescence microscopy.
- Deletion mapping to identify domains responsible for nuclear localization and speckling.
- Co-expression experiments to assess heterodimerization effects on localization.
Main Results:
- Individually, c-Myc and Mad proteins localized to subnuclear speckles, while Max showed a homogeneous nuclear pattern.
- Protein distributions were co-dominant and dynamic, adopting the pattern of the heterodimeric partner upon co-expression.
- Domains for nuclear localization and speckling were separable in Mad proteins (Mad1, Mxi1).
- A non-speckling Mxi1 mutant exhibited reduced transcriptional repressor activity.
- c-Myc-Max heterodimers, unlike Max-Mad, co-localized with SC-35 domains involved in mRNA processing.
Conclusions:
- Myc network proteins form dynamic subnuclear structures that are influenced by heterodimerization.
- c-Myc-Max heterodimers associate with active pre-mRNA transcription/processing sites, suggesting a role in transcriptional activation.
- Max-Mad heterodimers do not show this association, implying distinct regulatory mechanisms for repression.
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